SONOS (silicon-oxide-nitride-oxide-silicon) is a type of nonvolatile trapped-charge semiconductor memory technology. A SONOS memory cell can be fabricated as an N-type or P-type MOSFET (metal-oxide-semiconductor field effect transistor) with additional layers of oxide between the channel and the gate.
In its intrinsic state, the SONOS transistor operates like a conventional IGFET. The device is normally off (nonconductive) when a voltage (VDs) is applied between the drain and source terminals. If a voltage of the proper polarity and magnitude (the “threshold” voltage, VTH) is applied between the gate and source terminals, the device may turn on and conduct current between the drain and source (IDs). The intrinsic threshold voltage is negative for a P-type device and positive for an N-type device. The intrinsic threshold voltage of a SONOS device is a function of the impurity (dopant) levels in the channel region. For an NSONOS device with typical dopant levels, the intrinsic threshold voltage, VTH, is typically in the range of 0.4 to 0.8 volts. However, data retention in such a device may still break down over time.